This invention relates to improved magnetic recording media of the type are known as discs and is particularly advantageous when embodied in those flexible disc products known as floppy discs.
In reading the following discussion of the background of the invention, it must be remembered that the discussion has been prepared with a full knowledge of the invention. It cannot be, and is not intended to be, a view of the existing art as apparent to one or ordinary skill in the art at a time preceding the invention.
The magnetic recording members, subject of this invention, comprise a coating of magnetic particles in a matrix such as an organic polymer matrix. There are two general kinds of such members. A first kind is typified by the most common type of magnetic tape. This kind of product moves in a longitudinal direction in relationship to a recording or reading head because information is recorded sequentially along the length of the medium. The other kind of product is a disc which rotates while the reading head is positioned radially or helically, much in the relationship of a needle on a phongraph disc. These products are very well recognized in the art and have each been in extensive commercial use for some time. There has been a great deal of effort expended over the last thirty years or so to improve the magnetic recording characteristics of such products. One such procedure has been the orientation of the magnetic particles within their matrix. A discussion of such orientation is presented in The Physics of Magnetic Recording by C. D. Mee; 1968; North-Holland Publishing Company - Amsterdam. Both longitudinal and vertical orientation is discussed. Longitudinal orientation is orientation along the direction of travel of the recording member, e.g. along a tape. Vertical orientation would be the same particles "standing up" on end, i.e. normal to the tape surface. Mee points out that use of orienting techniques is not indicated for applications such as magnetic discs "which require tape to be magnetized in different directions". Among additional publications disclosed various orientation techniques and procedures are U.S. Pat. Nos. 3,052,567 to Gabor et al; 3,185,755; 2,796,359; 3,256,112; 3,117,065; 2,711,901; 3,261,706; 3,065,105 and 3,627,680. U.S. Pat. Nos. 3,117,065; 3,052,567 and 3,185,775 discuss vertical orientation of magnetic particles. There is somewhat related art, typified by Patent 3,001,891, wherein an AC orienting field is used to allow a "free" orientation of particles in response to other fields.
In some known art, discs have included oriented particles. This is especially true of inadvertent radial orientation of rigid (as opposed to floppy) spin-coated magnetic discs. Also, U.S. Pat. Nos. 3,256,112 and 3,001,891 disclose the use of an orienting procedure and state that discs may be oriented. In such a procedure, the particles have always been oriented in a circular direction, i.e. in a direction directly analogous to the longitudinal direction in tapes. It is believed that not even this type of procedure has been sufficiently advantageous to be commercially adopted in disc manufacture.
The flexible articles known as floppy discs are formed on a coating apparatus exactly like magnetic tape, then cut into circular discs. Such discs carry unoriented particles and cannot take advantage of any high magnetic squareness attributes of said particles. One example of such a disc format is described in "IBM Diskette, Original Equipment Manufacturers' Information" Product Reference Literature Number GA21-9190-1, File GENL-19 available from IBM Corporation.
In still another area of the magnetic recording art, there has been a great deal of attention being paid to the development of superior magnetic particles. Usually such particles are cobalt-based metal particles like those disclosed in U.S. Pat. No. 3,909,240 to Deffeyes et al; U.S. Pat. No. 3,574,685 to Haines et al and U.S. Pat. No. 3,607,218 to Akashi et al. Such particles can be utilized in providing much superior magnetic recording members than are possible using the traditional iron oxide particles. However, it is difficult to form magnetic recording members which fully exploit the potential of the improved metallic particles on commercial mixing coating equipment operated at acceptable output rates.
A major problem, in this respect, is the adequate dispersal of such powders. A number of inventors have provided improved means for dispersing such particles more efficiently. For example, U.S. Pat. No. 3,026,215 suggests the lengthwise and breadthwise orientation of such particles. Manly, in U.S. Pat. No. 3,172,776 discloses a process wherein chains of metal particles are formed to facilitate their being adapted more readily to an orienting procedure. A somewhat similar suggestion is made in U.S. Pat. No. 3,228,882 to Harle et al. Akashi et al, in U.S. Pat. No. 3,740,266 suggests still another method for overcoming problems inherent in orienting the more acicular of said particles.
While the problem of coating defects has slowed down the commercial application of such metal powders in magnetic tape, it has been a particular barrier to using the powders in floppy discs wherein a defect in the coating will necessarily cause a large area of processed tape to be discarded.
In the disclosure set forth below, the inventors will describe an improved magnetic disc and process for making the same. The novel disc product embodies attributes substantially overcoming the limitations of the disc products described above. Applicants, in solving the problem associated with such discs, have discovered a general solution to the roping problems heretofore associated with both acicular particles and small metallic particles, a solution useful in both disc and tape products. Also of interest in the prior art are polymodal materials are disclosed in copending and commonly owned U.S. Patent Application Ser. No. 411,253 filed Oct. 31, 1973 by Deffeyes.
That application, and Belgian Patent 823,013, incorporated by reference herein, disclose a system based on the use of a magnetic identifiction medium formed of at least two distinct populations of ferromagnetic powders, wherein a first population is selected so that it can magnetically switched, i.e. recorded upon, by a magnetic field at which a second population will not be switched. Each population is also responsive to differing stimuli, say differing magnetic fields or temperatures, for erasing information therefrom; that is magnetic information can be retained by one population as magnetic information is erased from another population. In general, it is desirable to have populations in a bimodal system characterized by coercivity values have a difference of at least 200. In practice, bimodal systems may be selected to have differences of 1000 or greater. If additional functionality is built into a ferromagnetic system by adding different modes, the higher and lower coerivities will usually differ by at least a factor of 200 (X-1) wherein X is the number of modes and it will be advantageous to keep the coercive force difference between the different populations at 200 oersteds or more.
The discovery that certain combinations of powders can be utilized in achieving such distinctly polymodal ferromagnetic systems was unexpected in view of the performance of mixtures formed of the ferromagnetic powders generally used in the art. For example, a mixture formed of a first iron oxide, a second ion oxide and cobalt-based metal powders having coercivities of 180, 320 and 1000, respectively, yielded a lowcoercivity peak which was not distinct although there were distinct peaks between the low coercivity materials and the high coercivity material.
The above result may be partly understood in view of the teaching that the peaks should be about 200 oersteds apart. However, the two low coercivity powders interact more than would be expected on the sole basis of the insufficient difference in coercivities.
In general, it appears that the metal powders, i.e. those of Bm values exceeding about 8000 gauss are the most advantageous for use in forming bimodal systems, in that they can be used successfully in combination with oxides and other metal powders.
Some typical Bm values are 3400 for gamma Fe.sub.2 O.sub.3, 4000 black Fe.sub.3 O.sub.4, about 16,000 for the cobalt metal powder of Example 1 below and about 13,000 for the low-coercive force metal powder of Examples 2 and 3.
In general, the peaks visible on a non-integrated dM/dt curve of an advantageous polymodal system can usually be recognized in two ways:
1. As one raises the field on a BH meter the lower-coercivity curves will be substantially complete before the appearance of the next higher curve.
2. The valleys between adjacent peaks are advantageously of a depth equal to at least one half of the average height of the adjacent peaks over a base line.